The tank in question is a 10gal with 6 zebra danios, 2 spotted cories, a pleco, and 2 apple snails. It is heated and filtered and has a bubbler. I do 50% water changes with gravel vac whenever I think it needs it (like once a week) and 100% changes when the substrate gets substantially dirty. I use a double dose of Stress Coat + as my water conditioner.

Okay, now my story:

A few days ago, I brought home a new betta from Petco. I acclimated him and put him in the 10gal and everything seemed just fine. He was active, eating, and swimming around just fine. The next day, he's lying on the bottom of the tank breathing heavily. I took him out and put him back in his cup to make it easier for him to get to the surface. Within an hour, he was dead. Since he was new and from Petco, I wasn't really surprised, so I thought nothing of it.

The next day, I took my betta that I had in a 1gal and put him in the 10gal, thinking he'd like the space. I've had this betta in the 1gal for months and he has been perfectly healthy. He goes in the 10gal and everything is fine. He's active, exploring, eating, and acting perfectly normal. A day or two later, he's lying on his side on the bottom of the tank breathing heavily. Again, I put him in a cup and he's soon dead. This was a betta that was perfectly healthy for months. Also, something I find interesting is that during all of this, none of the other fish in the 10gal have been affected in any way. They are all still active, playing with each other, and eating just fine. I'm so confused..

Can I just say that your 10 gallon is overstocked. If it is a common plecos that can grow to at least a foot, and zebra danios need a larger tank as they are extremely active fish.

Do you have any parameters for that tank? If it has a filter running it may be cycled but if not it could be that your water quality is bad.

Sometimes fish can acclimatise to poor water quality and live in these conditions for years. However, whenever any new fish are introduced to the tank they sicken and/or die because they cannot adjust to it so quickly.

I understand that my tank may be a little overstocked (thanks Petco fish workers) but I try to keep my water very clean. Even if it was poor water quality that killed the first betta, why would it kill my other betta who had been living in the exact same water in a smaller tank for months?

Thanks for the reply. I have an ammonia test kit that I can use when I get home. I had been changing the water in the 1gal about 100% weekly, so if anything, he was moved to cleaner water. The temp. may have been warmer but I acclimated him to it. Both tanks have heaters, but the 1gal usually stays 74/78 while the 10gal stays a toasty 80/82. I acclimated him by putting him in his Petco cup in his old tank water and floating it in the new tank for an hour, putting new tank water in every 15min. He seemed very happy the first day, then he just went downhill just like the betta from Petco. What could have happened? I don't think it's ammonia poisoning or anything like that because none of the other fish or snails were affected at all. I really wish I could find out what happened, because I'd like to put another betta in that tank..

He could have been super stressed from the zebra danios zooming around everywhere. Unfortunately danios aren't really good for anything smaller than a 20 gallon or high tanks because they love to zoom everywhere.

And to reiterate what LittleBettaFish said is that your danios, pleco, cories and snails could be accustomed to the ammonia levels in there, so thus they wouldn't seem "effected" by it but then when you put the betta in, it would be a drastic difference and eventually kill them.